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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::map&lt;Key,T,Compare,Allocator&gt;::</span>begin, <span style="font-size:0.7em; line-height:130%">std::map&lt;Key,T,Compare,Allocator&gt;::</span>cbegin</h1>
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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="erase.html" title="cpp/container/map/erase"><span class="t-lines"><span>map::erase</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="swap.html" title="cpp/container/map/swap"><span class="t-lines"><span>map::swap</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="extract.html" title="cpp/container/map/extract"><span class="t-lines"><span>map::extract</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="merge.html" title="cpp/container/map/merge"><span class="t-lines"><span>map::merge</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
<tr class="t-nv-h2"><td colspan="5">查找</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="count.html" title="cpp/container/map/count"><span class="t-lines"><span>map::count</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="find.html" title="cpp/container/map/find"><span class="t-lines"><span>map::find</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="contains.html" title="cpp/container/map/contains"><span class="t-lines"><span>map::contains</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="equal_range.html" title="cpp/container/map/equal range"><span class="t-lines"><span>map::equal_range</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="lower_bound.html" title="cpp/container/map/lower bound"><span class="t-lines"><span>map::lower_bound</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="upper_bound.html" title="cpp/container/map/upper bound"><span class="t-lines"><span>map::upper_bound</span></span></a></div></div></td></tr>
<tr class="t-nv-h2"><td colspan="5">观察器</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="key_comp.html" title="cpp/container/map/key comp"><span class="t-lines"><span>map::key_comp</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="value_comp.html" title="cpp/container/map/value comp"><span class="t-lines"><span>map::value_comp</span></span></a></div></div></td></tr>
<tr class="t-nv-h1"><td colspan="5">非成员函数</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="swap2.html" title="cpp/container/map/swap2"><span class="t-lines"><span>std::swap</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="erase_if.html" title="cpp/container/map/erase if"><span class="t-lines"><span>erase_if</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="operator_cmp.html" title="cpp/container/map/operator cmp"><span class="t-lines"><span>operator==</span><span>operator!=</span><span>operator&lt;</span><span>operator&gt;</span><span>operator&lt;=</span><span>operator&gt;=</span><span>operator&lt;=&gt;</span></span></a></div><div><span class="t-lines"><span></span><span><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></span><span><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></span><span><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></span><span><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></span><span><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></span><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> <a href="deduction_guides.html" title="cpp/container/map/deduction guides">推导指引</a><span class="t-mark-rev t-since-cxx17">(C++17)</span></td></tr>
</table></div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<table class="t-dcl-begin"><tbody>
</tbody>
<tbody class="t-dcl-rev">
<tr class="t-dcl t-until-cxx11">
<td> <div><span class="mw-geshi cpp source-cpp">iterator begin<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-until-cxx11">(C++11 前)</span> </td>
</tr>
<tr class="t-dcl t-since-cxx11">
<td> <div><span class="mw-geshi cpp source-cpp">iterator begin<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-since-cxx11">(C++11 起)</span> </td>
</tr>
</tbody><tbody>
</tbody>
<tbody class="t-dcl-rev">
<tr class="t-dcl t-until-cxx11">
<td> <div><span class="mw-geshi cpp source-cpp">const_iterator begin<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-until-cxx11">(C++11 前)</span> </td>
</tr>
<tr class="t-dcl t-since-cxx11">
<td> <div><span class="mw-geshi cpp source-cpp">const_iterator begin<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-since-cxx11">(C++11 起)</span> </td>
</tr>
</tbody><tbody>
<tr class="t-dcl t-since-cxx11">
<td> <div><span class="mw-geshi cpp source-cpp">const_iterator cbegin<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-since-cxx11">(C++11 起)</span> </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<p>返回指向 <code>map</code> 首元素的迭代器。
</p><p>若 <code>map</code> 为空，则返回的迭代器将等于 <span class="t-lc"><a href="end.html" title="cpp/container/map/end">end()</a></span> 。
</p>
<div class="t-image">
<p><img alt="range-begin-end.svg" src="../../../../common/images/1/1b/range-begin-end.svg" width="546" height="157">
</p>
</div>
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
<p>（无）
</p>
<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
<p>指向首元素的迭代器。
</p>
<h3><span class="mw-headline" id=".E5.A4.8D.E6.9D.82.E5.BA.A6">复杂度</span></h3>
<p>常数。
</p><p><br>
</p>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;map&gt;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
  <a href="../map.html"><span class="kw1281">std::<span class="me2">map</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span>, <span class="kw4">float</span><span class="sy1">&gt;</span> num_map<span class="sy4">;</span>
  num_map<span class="br0">[</span><span class="nu0">4</span><span class="br0">]</span> <span class="sy1">=</span> <span class="nu16">4.13</span><span class="sy4">;</span>
  num_map<span class="br0">[</span><span class="nu0">9</span><span class="br0">]</span> <span class="sy1">=</span> <span class="nu16">9.24</span><span class="sy4">;</span>
  num_map<span class="br0">[</span><span class="nu0">1</span><span class="br0">]</span> <span class="sy1">=</span> <span class="nu16">1.09</span><span class="sy4">;</span>
  <span class="co1">// 调用 a_map.begin() 及 a_map.end()</span>
  <span class="kw1">for</span> <span class="br0">(</span><span class="kw4">auto</span> it <span class="sy1">=</span> num_map.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> it <span class="sy3">!</span><span class="sy1">=</span> num_map.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="sy2">++</span>it<span class="br0">)</span> <span class="br0">{</span>
    <a href="../../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> it<span class="sy2">-</span><span class="sy1">&gt;</span>first <span class="sy1">&lt;&lt;</span> <span class="st0">", "</span> <span class="sy1">&lt;&lt;</span> it<span class="sy2">-</span><span class="sy1">&gt;</span>second <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
  <span class="br0">}</span>
<span class="br0">}</span></pre></div></div>
<p>输出：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">1, 1.09
4, 4.13
9, 9.24</pre></div></div> 
</div>
<h4> <span class="mw-headline" id=".E4.BD.BF.E7.94.A8.E8.87.AA.E5.AE.9A.E4.B9.89.E6.AF.94.E8.BE.83.E5.87.BD.E6.95.B0.E7.9A.84.E7.A4.BA.E4.BE.8B">使用自定义比较函数的示例</span></h4>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;cmath&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;map&gt;</span>
 
<span class="kw1">struct</span> Point <span class="br0">{</span> <span class="kw4">double</span> x, y<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">typedef</span> Point <span class="sy2">*</span> PointPtr<span class="sy4">;</span>
<span class="co1">// 比较二个 Point 指针的 x 坐标和 y 坐标</span>
<span class="kw1">struct</span> PointCmp <span class="br0">{</span>
    <span class="kw4">bool</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span><span class="kw4">const</span> PointPtr <span class="sy3">&amp;</span>lhs, <span class="kw4">const</span> PointPtr <span class="sy3">&amp;</span>rhs<span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> 
        <span class="kw1">return</span> lhs<span class="sy2">-</span><span class="sy1">&gt;</span>x <span class="sy1">&lt;</span> rhs<span class="sy2">-</span><span class="sy1">&gt;</span>x<span class="sy4">;</span> 
    <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
    <span class="co1">// 注意即使 x 坐标不按顺序， map 仍将按 x 坐标升序迭代</span>
    Point points<span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span> <span class="br0">{</span><span class="nu0">2</span>, <span class="nu0">0</span><span class="br0">}</span>, <span class="br0">{</span><span class="nu0">1</span>, <span class="nu0">0</span><span class="br0">}</span>, <span class="br0">{</span><span class="nu0">3</span>, <span class="nu0">0</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
 
    <span class="co1">// mag 是将结点地址发送到其在 x-y 平面上的模的映射</span>
    <span class="co1">// 尽管关键是指针到 Point ，我们希望按 x 坐标顺序而非按 Point 地址的顺序。</span>
    <span class="co1">// 这通过使用 PointCmp 类的比较方法进行。</span>
    <a href="../map.html"><span class="kw1281">std::<span class="me2">map</span></span></a><span class="sy1">&lt;</span>Point <span class="sy2">*</span>, <span class="kw4">double</span>, PointCmp<span class="sy1">&gt;</span> mag<span class="br0">(</span><span class="br0">{</span>
        <span class="br0">{</span> points,     <span class="nu0">2</span> <span class="br0">}</span>,
        <span class="br0">{</span> points <span class="sy2">+</span> <span class="nu0">1</span>, <span class="nu0">1</span> <span class="br0">}</span>,
        <span class="br0">{</span> points <span class="sy2">+</span> <span class="nu0">2</span>, <span class="nu0">3</span> <span class="br0">}</span>
    <span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
 
    <span class="co1">// 从 0 更改每个 y 坐标为模</span>
    <span class="kw1">for</span><span class="br0">(</span><span class="kw4">auto</span> iter <span class="sy1">=</span> mag.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> iter <span class="sy3">!</span><span class="sy1">=</span> mag.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="sy2">++</span>iter<span class="br0">)</span><span class="br0">{</span>
        <span class="kw4">auto</span> cur <span class="sy1">=</span> iter<span class="sy2">-</span><span class="sy1">&gt;</span>first<span class="sy4">;</span> <span class="co1">// 指向 Node 的指针</span>
        cur<span class="sy2">-</span><span class="sy1">&gt;</span>y <span class="sy1">=</span> mag<span class="br0">[</span>cur<span class="br0">]</span><span class="sy4">;</span> <span class="co1">// 亦可使用 cur-&gt;y = iter-&gt;second;</span>
    <span class="br0">}</span>
 
    <span class="co1">// 更新并打印每个结点的模</span>
    <span class="kw1">for</span><span class="br0">(</span><span class="kw4">auto</span> iter <span class="sy1">=</span> mag.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> iter <span class="sy3">!</span><span class="sy1">=</span> mag.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="sy2">++</span>iter<span class="br0">)</span><span class="br0">{</span>
        <span class="kw4">auto</span> cur <span class="sy1">=</span> iter<span class="sy2">-</span><span class="sy1">&gt;</span>first<span class="sy4">;</span>
        mag<span class="br0">[</span>cur<span class="br0">]</span> <span class="sy1">=</span> <a href="../../numeric/math/hypot.html"><span class="kw1331">std::<span class="me2">hypot</span></span></a><span class="br0">(</span>cur<span class="sy2">-</span><span class="sy1">&gt;</span>x, cur<span class="sy2">-</span><span class="sy1">&gt;</span>y<span class="br0">)</span><span class="sy4">;</span>
        <a href="../../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"The magnitude of ("</span> <span class="sy1">&lt;&lt;</span> cur<span class="sy2">-</span><span class="sy1">&gt;</span>x <span class="sy1">&lt;&lt;</span> <span class="st0">", "</span> <span class="sy1">&lt;&lt;</span> cur<span class="sy2">-</span><span class="sy1">&gt;</span>y <span class="sy1">&lt;&lt;</span> <span class="st0">") is "</span><span class="sy4">;</span>
        <a href="../../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> iter<span class="sy2">-</span><span class="sy1">&gt;</span>second <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
    <span class="br0">}</span>
 
    <span class="co1">// 以基于范围的 for 循环重复以上内容</span>
    <span class="kw1">for</span><span class="br0">(</span><span class="kw4">auto</span> i <span class="sy4">:</span> mag<span class="br0">)</span> <span class="br0">{</span>
        <span class="kw4">auto</span> cur <span class="sy1">=</span> i.<span class="me1">first</span><span class="sy4">;</span>
        cur<span class="sy2">-</span><span class="sy1">&gt;</span>y <span class="sy1">=</span> i.<span class="me1">second</span><span class="sy4">;</span>
        mag<span class="br0">[</span>cur<span class="br0">]</span> <span class="sy1">=</span> <a href="../../numeric/math/hypot.html"><span class="kw1331">std::<span class="me2">hypot</span></span></a><span class="br0">(</span>cur<span class="sy2">-</span><span class="sy1">&gt;</span>x, cur<span class="sy2">-</span><span class="sy1">&gt;</span>y<span class="br0">)</span><span class="sy4">;</span>
        <a href="../../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"The magnitude of ("</span> <span class="sy1">&lt;&lt;</span> cur<span class="sy2">-</span><span class="sy1">&gt;</span>x <span class="sy1">&lt;&lt;</span> <span class="st0">", "</span> <span class="sy1">&lt;&lt;</span> cur<span class="sy2">-</span><span class="sy1">&gt;</span>y <span class="sy1">&lt;&lt;</span> <span class="st0">") is "</span><span class="sy4">;</span>
        <a href="../../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> mag<span class="br0">[</span>cur<span class="br0">]</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
        <span class="co1">// 注意与 std::cout &lt;&lt; iter-&gt;second &lt;&lt; '\n'; 相反，上述的 </span>
        <span class="co1">// std::cout &lt;&lt; i.second &lt;&lt; '\n'; 不会打印更新的模</span>
    <span class="br0">}</span>
<span class="br0">}</span></pre></div></div>
<p>输出：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">The magnitude of (1, 2) is 2.23607
The magnitude of (2, 1) is 2.23607
The magnitude of (3, 3) is 4.24264
The magnitude of (1, 2.23607) is 2.44949
The magnitude of (2, 2.23607) is 3
The magnitude of (3, 4.24264) is 5.19615</pre></div></div> 
</div>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<table class="t-dsc-begin">

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="end.html" title="cpp/container/map/end"> <span class="t-lines"><span>end </span><span>cend</span></span></a></div><div><span class="t-lines"><span></span><span></span></span></div></div>
</td>
<td>  返回指向容器尾端的迭代器  <br> <span class="t-mark">(公开成员函数)</span> </td></tr>
</table>

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